Exemplar, in the sense developed by philosopher of scienceThomas Kuhn, is a well known usage of a scientific theory.
According to Kuhn, scientific practice alternates between periods of normal science and extraordinary/revolutionary science. During periods of normalcy, scientists tend to subscribe to a large body of interconnecting knowledge, methods, and assumptions which make up the reigning paradigm (see paradigm shift for more information on Kuhn's model). Normal science presents a series of "puzzles" that are solved as scientists explore their field. The solution to some of these puzzles become well known and are the exemplars of the field.

"[b]ecause the term [paradigm] has assumed a life of its own ... I shall here substitute ‘exemplars.’ By it I mean, initially, the concrete problem-solutions that students encounter from the start of their scientific education, whether in laboratories, on examinations, or at the ends of chapters in science texts. ... All physicists, for example, begin by learning the same exemplars: problems such as the inclined plane, the conical pendulum, and Kepler's laws of planetary motion instruments such as the vernier, the calorimeter, and the Wheatstone bridge."

Those who study a scientific discipline are expected to know its exemplars. There is no fixed set of exemplars, but for a physicist today it would certainly include such things as the harmonic oscillator from mechanics and the hydrogen atom from quantum mechanics. For a biologist today the set includes the population variations of the European peppered moth (Biston betularia) and the convergent evolution of wings.